1. Field of the Invention
The present invention relates to a color image reading apparatus in which a color original is exposed and the transmitted or reflected light therefrom is guided to imaging devices such as charge-coupled devices for achieving color-separated reading of said original.
2. Description of the Prior Art
In known color image reading apparatus in which an original is illuminated with a light source and the reflected light therefrom is color separated with color separation filters and read with imaging devices such as charge-coupled devices (CCD's) it is difficult to maintain a constant ratio among image signal levels of different colors due, for example, to uneven spectral distribution of the emission from the light source. As an example, in case of a halogen lamp which is commonly used as the light source in such information reading apparatus, the output signals from a CCD become greater in the order of blue (B), green (G) and red (R) as shown in FIG. 1. Thus, as the output levels of the image signals from the imaging device increase in the above-mentioned order in case a color original is illuminated with a single light source, it is not possible to expect an exact reproduction of the relative levels of different colors on the original. The above-mentioned spectral distribution of the light source, for example of a halogen lamp, also varies with the lapse of time. Besides the color balance in a color image reading apparatus is affected by the relative difference in the spectral sensitivities of a CCD in the regions of blue, green, red and near-infrared, cut-off wavelengths of color separation filters, the difference in the transmission levels of said filters in the regions of blue, green and red, and the difference in the near-infrared cut-off wavelength and the visible light transmission of a near-infrared cut-off filter used for improving the color separating function by reducing the near-infrared transmission of the color separation filters. Particularly in case of reproducing a color image by connecting a color image reading apparatus with a color laser beam printer, satisfactory image quality cannot be obtained even with a slight difference in the output signal levels of different colors, due to aberrated color balance among the images of different colors.
In order to adjust the color balance it has been proposed to supply the light source with a manually determined control voltage for each color-separated exposure, but such method is not only complicated and difficult but also is not effective for the aberration or fluctuation of the sensitivity of a photoelectric converting element such as a CCD and of various sensors.